WFS Feature / Zoom to filtered results.

[fxprunayre]

When indexing features, add 4 fields to store the coordinate of the
bounding box of each features. On client side use min/max aggregations
to retrieve the bounding box of all features matching filter.

When ES support for geo_bounds aggregation will be implemented for geo_shape type
See https://github.com/elastic/elasticsearch/issues/7574
The following would be better probably:

```
 "viewport" : {
   "geo_bounds" : {
     "field" : "location",
     "wrap_longitude" : true
   }
 }
```

See in action https://youtu.be/SKwnwN5RPKA

[fxprunayre]

Has been waiting long for a review. It has been live tested for 2 months on projects. Merging.